This invention relates generally to a corrosive-resistant composite head fastener and more particularly to plastic coated head on a self-drilling or tapping fastener.
Prior art devices of the general catagory are primarily used to provide corrosive or weather resistance to threaded fasteners in the building construction field. The use of plastic on the heads of such fasteners enables the fasteners to also be color coordinated with a variety of colors of exterior building panels. Prior art composite headed fasteners typically cap, mold or otherwise form plastic material over an enlarged metal head to provide external wrenching surfaces for association with a socket-type driving tool. Since plastic material is weak and will fail in shear and/or compression, the prior art devices are deficient in their ability to transmit the high torque requirements in a self-tapping or drilling environment to the threaded shank. The excessive torques required to seat such a fastener tend to localize or be concentrated at the edges of the external plastic wrenching surfaces, causing the plastic to rupture, break or round over prior to reaching the desired torque.
Certain other prior art devices may not fail in their extenal surfaces but still are not capable of withstanding high torques because the plastic fails to interlock effectively with the underlying head, thus permitting the plastic to spin relative to the metal head.
While plastic headed fasteners of the prior art theoretically provide color matching, sealing and corrosive resistant characteristics, they have outstanding deficiencies in failure prior to seating or completely installing the fastener.